1. Field of the Invention
The present invention is directed to a voltage comparison circuit capable of detecting that two different signals reach a certain offset level and used for suppressing noise in a differential serial signal and detecting connection to transmission lines used for transmitting a differential serial signal. The present invention, in particular, relates to a voltage comparison circuit having an offset, applicable to a squelch circuit and a disconnection detection circuit, for example, used in USB 2.0, and also applicable for Hall signal detection by a hysteresis comparator used in a motor driver.
2. Description of the Related Art
A conventional voltage comparison circuit having an offset uses a method of setting the value of the offset voltage by connecting load resistors to the source terminals of transistors forming a differential pair, as shown in FIG. 11 (for example, see Patent Document 1). In addition, as a means to vary the offset voltage value, MOS switches for changing the resistance value are used or laser trimming is performed, to improve the accuracy of the setting of the offset voltage value. Another conventional method sets the offset voltage value by controlling the current value of one terminal of a constant current source load (e.g. see Patent Document 2).    [Patent Document 1] Japanese Laid-open Patent Application Publication No. 2004-194124    [Patent Document 2] Japanese Patent Publication No. 3926645
However, in the case illustrated in FIG. 11, it is necessary to use a resistor large enough to be able to ignore the on-resistance of the MOS switches or the trimming bit resistor of the laser trimming, and therefore, the amount of current allowed to flow through the differential pair is limited. Accordingly, the conventional technology is unsuitable for high-speed response detection capable of detecting the level difference of two signals in compliance with high-speed serial transmission, for example, USB 2.0 serial data link. Also, resistors and MOS switches relatively large in size are required, leading to an increase in the circuit size. Furthermore, in the case of setting the offset voltage by switching the MOS switches or by laser trimming, the setting needs to be made in a post-manufacturing process, which leads to an increase in cost. The case of setting the offset voltage by controlling the current value of one terminal of a constant current source load is suitable for high-speed serial transmission since it allows high-speed response; however, if matching of respective transistors is not ideal, the accuracy of the setting of the offset voltage value varies to an extent, thereby making it difficult to control the current value. Furthermore, if the detection offset level of a differential signal is large, the current ratio between the transistors of the differential pair becomes extremely large, making it difficult to control the variation.